1. Field of the Invention
The present invention relates to a lamp driving circuit in a vehicle turn/hazard system, and more particularly to a lamp driving circuit in a vehicle turn/hazard system which can recognize the blinking operation of a lamp on the basis of a switching sound of a relay in such a manner that the blinking of the lamp is on/off controlled by the relay.
2. Description of the Related Art
Such a previously known lamp driving circuit is shown in FIG. 8. As seen from FIG. 8, a battery power source Vcc is connected to a turn/hazard lamp 10 through a relay contact C of a relay RY. In operation, as seen from FIG. 9A, after the battery is turned on, when a turn switch 14a or hazard switch 14 connected to a control unit 12 is turned on, the control unit 12 produces a pulse signal having a prescribed period corresponding to the blinking period of the turn/hazard lamp 10.
The pulse signal produced by the control unit 12 is applied to the base of a transistor 16 so that the transistor 16 is turned on/off to energize a relay coil L. Thus, as shown in FIG. 9B, the relay contact C is switched from the off state into the on state, or vice versa. A current is supplied intermittently from the battery to the turn/hazard lamp 10 so that the lamp 10 blinks. Incidentally, the relay RY is used in the lamp driving circuit so that the blinking operation of the lamp 10 can be recognized by the switching sound of the relay.
In the configuration described above, as seen from FIGS. 9B and 9C, at timings t1 and t3, the relay contact C turns on and hence the turn/hazard lamp 10 also turns on. At this time, as seen from FIG. 9D, a large rushing current is passed through the turn/hazard lamp 10. This rushing current is also supplied to the relay contact C. However, at the timing when the rushing current has been supplied to the relay contact C, the relay contact is not still in the stable conducting state. Further, at timings t2 and t4, when the relay contact C turns off and hence the turn hazard lamp 10 turns off, an arc is generated at the contact because the voltage has been applied to the contact C from the battery.
If the large rushing current flows through the relay contact C which is not in the stable conducting state, the arc occurs at the relay contact C, the relay contact C is exhausted. The repetition of such a phenomenon for a long time leads to the faulty contact of the relay contact. As a result, the lamp does not operate normally. However, in the turn/hazard system, the relay contact is switched with a high frequency. Therefore, the relay is required to have high endurance to its repetitive switching. In order to satisfy such a requirement, a system can be proposed which uses a high-class expensive relay or can easily replace the broken relay by a new one. However, this leads to high production cost of the system.